The invention relates generally to the use of a particular amount of non-condensible inert gas as a stripping medium in deodorizing edible oils and/or fats and more particularly to the use of substantially less than the theoretically required amount of nitrogen as a stripping medium in deodorizing edible oils and/or fats.
Deodorization is usually the final processing step in the production of edible oil and fat products. Commonly, edible oils or fats are subject to either chemical refining involving degumming, neutralizing, dewaxing, washing and filtrating steps or physical refining involving degumming, decoloring and filtering steps, prior to deodorization. The type of refining involved, i.e. chemical or physical refining, could dictate the operating conditions of deodorization. Severe deodorization operating conditions, for example, may be necessary to obtain edible oil and fat products having the desired characteristics when physical refining, as opposed to chemical refining, is employed prior to deodorization. The physical refining is likely to produce edible oils or fats having a greater amount of impurities than those produced by chemical refining due to the limited refining steps involved.
Deodorization basically involves stripping edible oils and/or fats to remove, among other things, substances that impart disagreeable odor and taste. The substances removed usually include free fatty acids; various disagreeable odor and taste causing compounds, such as aldehydes, ketones, alcohols and hydrocarbons; and various compounds formed by the heat decomposition of peroxides and pigments. These substances should be sufficiently removed to impart the desired property to the edible oil and/or fat. The fatty acids in the edible oils and/or fats, for example, should be substantially reduced, to about 0.1 to 0.2% to obtain the edible oil and/or fat having the desired properties.
During deodorization vapors are formed as a result of stripping the edible oils and/or fats with inert stripping gas at a high temperature condition. These vapors which contain valuable by-products, such as fatty acid and other impurities, can pose problems in the standpoint of waste disposal. The vapors are, therefore, usually condensed to produce condensates having valuable by-products. The condensation, like deodorization, is generally accomplished under high vacuum which may be generated by vacuum boosters and/or ejectors supplied with steam (motive steam). Motive steam employed to generate high vacuum, however, is contaminated by the vaporized impurities passing through the boosters and ejectors and needs to be treated before it can be disposed. The motive steam could, therefore, esculate the cost involved in operating deodorization systems unless its consumption can be reduced.
It has been known to employ steam (process steam) as a stripping gas in many deodorization systems. Process steam is suitable as a deodorizing stripping gas because of its high specific volume, inexpensiveness and easily condensable and removable characteristics. The amount of process steam theoretically necessary to maximize stripping may be determined by the following formula: ##EQU1## S=molar flow rate of the stripping steam Pv=vapor pressure of the free fatty acid
P=total system pressure PA1 C=molar concentration of fatty acid in the oil PA1 M=total number of moles of edible oil and/or fat PA1 E=vaporization efficiency PA1 Ac=activity coefficient PA1 C*=Fatty acid in the oil at equilibrium PA1 Ci=initial molar concentration of free fatty acid PA1 Cf=final molar concentration of fatty acid PA1 Pa*=Equilibrium partial pressure of free fatty acid and other contaminants PA1 P.sub.t =Total pressure PA1 R=Gas constant PA1 T=Absolute Temperature PA1 P=Gas pressure PA1 V.sub.steam =Total volume of steam
Commercially, the amount of process steam employed to maximize stripping is generally about 34 lb to about 39.6 lb of process steam per ton of edible oil or fat. In spite of the minimum amount of process steam involved, however, in removing the optimum amount of impurities in the edible oil and/or fat, motive steam consumption remains high. In addition, the use of process steam may lead to the reduction of deodorized edible oil and/or fat products. Commercial deodorization systems employing about 34 lb to 39.6 lb of process steam per ton of edible oil and/or fat, for example, may lose up to about 0.5% by weight of edible oil and/or fat due to the entrainment and unwanted side reactions such as thermal decomposition and possibly hydrolysis reaction. The above problems are further compounded by the formation of a condensate containing a low percentage of fatty acid which results from cooling the vapor formed during steam deodorization. The condensate, due to its low fatty acid content, needs to be treated further in distillation equipment or needs to be disposed as a waste stream or as an animal feed after it is treated to remove all pollutants or contaminents.
As a result of the problems inherent in deodorization systems which employ process steam as a stripping gas, the use of nitrogen or other inert gas, in lieu of steam, as a stripping medium has been considered. Theoretically, equal molar of nitrogen or other inert gas is needed to replace equal molar of steam in deodorizing edible oils and/or fats. That is, equal moles of nitrogen or inert gas is theoretically needed to replace steam in order to carry the same amount of volatile or impurities as steam. The necessity for this theoretically required equal mole of nitrogen or other inert gas is expressed in terms of the thermodynamic relationship governing the removal of free fatty acid and other contaminents in the edible oils and/or fats: ##EQU2## where Ya=Equilibrium mole fraction of free fatty acid and other contaminants in the gas phase per mole of stripping gas.
As the equilibrium mole fraction of the free fatty acid in the gas phase increases, there is a higher tendency that the free fatty acid will be removed from the oil. The total moles of free fatty acid and other contaminants which can be removed at equilibrium conditions, are therefore defined by: EQU M.sub.T =Ya M.sub.steam ( 2)
where M.sub.T =Total moles of free fatty acid and contaminating volatile removed.
M.sub.steam =Total moles of steam used The volume of nitrogen or other inert gas, however, may be calculated using ideal gas law since the deodorization system operates under vacuum. EQU M.sub.steam =TR/PV.sub.steam ( 3)
where
It then logically follows that, by theory, equal volume or equal moles of nitrogen or other inert gas is required to replace equal volume or moles of steam in deodorizing edible oil and/or fat. Unfortunately, the use of the theoretical amount or equal moles of non-condensible nitrogen or other inert gases, in lieu of steam, as a stripping medium increases motive steam consumption as a result of passing an excessive amount of non-condensable inert gas to vacuum boosters and ejectors. Moreover, an increased amount of cooling water may be needed to condense the vapor formed during deodorization since the cooling system involved could be overloaded with an excessive amount of non-condensible inert gas. Indeed, "Refining of Oils and Fats for Edible Purposes", written by Andersen and published by Pergamon Press, The Macmillan Co., New York, teaches away from using a non-condensible gas, in lieu of steam, because of the difficulties involved in removing and recovering the non-condensible inert gas.
It is an advantage of the present invention in reducing any difficulties involved in using the non-condensible inert gas in deodorization systems.
It is another advantage of the present invention in reducing the required amount of motive steam and cooling water without compromising the quality of deodorized edible oils and/or fats.
It is yet another advantage of the present invention in increasing the fatty acid content in the recovered condensates.
It is a further advantage of the present invention in improving the stability of deodorized edible oils and/or fats.
It is an additional advantage of the present invention in increasing the yield of deodorized edible oils and or fats by reducing the entrainment of deodorized edible oil and/or fat by a stripping medium and by inhibiting side reactions which may be responsible for the formation of some impurities.
The above and other advantages will become apparent to one skilled in the art upon reading this disclosure.